docs: def getYadisXRD(xrd_tree):
      """Return the XRD element that should contain the Yadis services"""
      xrd = None
  
      # for the side-effect of assigning the last one in the list to the
      # xrd variable
      for xrd in xrd_tree.findall(xrd_tag):
          pass
  
      # There were no elements found, or else xrd would be set to the
      # last one
      if xrd is None:
          raise XRDSError('No XRD present in tree')
  
      return xrd